An example of a non-destructive inspection apparatus is disclosed in U.S. Pat. No. 7,848,894, “Non-destructive Inspection Apparatus,” Dec. 7, 2010, which is hereby incorporated by reference in its entirety. The disclosed NDI apparatus includes an inertial sensor and an array including a plurality of elements for transmitting and receiving inspection signals towards and from the structure being inspected. More specifically, the disclosed system employs an inertial sensor to determine position and orientation of an NDI sensor when doing non-destructive scanning of a surface or structure, e.g., the structure of an in-service aircraft.
Inertial sensors do not directly measure position and orientation. Instead, they sense acceleration and the angular rotation rate. Acceleration can be integrated to determine velocity. The velocity can then be integrated to determine position. The angular rotation rate can be integrated to determine orientation. The position and orientation values may be used to create an image of the structure under inspection using the measured NDI data. However, a problem with this approach is that, due to this double integration, uncertainties in position accumulate over time. The uncertain position information can distort the resulting image and render it unusable. A goal of the present invention is to provide a commercially feasible solution to this problem with NDI systems.